1. Field of the Invention
The present invention relates to a process for preparing hard and porous polyvinylalcoh ol gels. More particularly, the invention relates to a process for the preparation of hard polyvinylalcohol gels for aqueous gel permeation chromatography.
2. Description of the Prior Art
Polystyrene gels and polyvinyl acetate gels are now used as a column packing material for gel permeation chromatography (GPC) using organic solvents. Polyacrylamide gels, dextran gels, starch gels and agar gels are used as the packing material for GPC using aqueous solvents. The packing material used with organic solvents are hard porous gels. However, the packing material used with aqueous solvents become soft as the pore size is increased. This is a great hindrance to enhancing the operation speed in aqueous GPC.
Herein, the term "hard gel" refers to a gel having a structure in which the pore size is large (for example, 100 to 100,000 A) for the gel in the dry state. Moreover, this large pore size is not significantly changed by swelling of the gel. In contrast, for a "soft gel", the pore size is very small in the dry state and is made larger when the gel is swollen with the aqueous solvent.
The cause of the softness of the packing material for aqueous GPC resides mainly in the packing material-preparation process. The pore structure of these packing materials is different from the pore structure of the packing material for organic GPC.
For preparation of the packing material for organic GPC, an organic diluent is added to a monomer and a crosslinking agent. The mixture is subjected to suspension polymerization in an aqueous medium. Pores are formed at those parts from which the diluent has escaped. Therefore, the degree of crosslinking and the pore size can be controlled easily, and a hard gel can be obtained. For preparation of the packing material for aqueous GPC, however, a water-soluble polymer is used as the starting material and is subjected to post-crosslinking to form pores. The pore size is controlled by adjusting the molecular weight of the starting polymer and the proportion of the crosslinking agent. That is, if the amount of the crosslinking agent is reduced, the size of the pores formed by crosslinking after swelling is increased. Accordingly, it is impossible to increase the pore size while retaining the hardness of the gel. Therefore, only soft gels are now available.
Polyvinylalcohol gels have been proposed as the packing material for aqueous GPC, and two processes for production of such polyvinylalcohol gels are known in the art. However, each of these polyvinylalcohol gels suffer from the defects mentioned above for the conventional packing material for aqueous GPC.
More specifically, according to one process, a soft gel is prepared by inverse suspension polymerization of polyvinylalcohol by using epichlorohydrin as a crosslinking agent; and according to the other process, monomeric vinyl acetate is polymerized in the presence of the sole additive, glycidyl methacrylate as a crosslinking agent. The resulting polymer is saponified with an alkali to form a polyvinylalcohol gel. According to these processes, however, it is still impossible to appropriately control both the strength of the gel and the pore size in combination. The products obtained by these processes are no better than the conventional soft gels and a need continues to exist for desirably hard gels for use in aqueous GPC.